Modern carving or parabolic snow ski can be easily turned by applying leg pressure on the edges which is transmitted through the camber shape to the front and rear part of the concave side cut. Grass ski or in general dry surface ski is designed to simulate snow ski. The most commonly used grass ski design is based on the rolling tread disclosed in U.S. Pat. No. 4,572,528. This design although ensures smooth riding makes turning quite difficult. Tight turns, critical for speed control, are very hard to achieve making such device unsafe and unsuitable for most recreational skiers. This type of ski is mainly used for racing and summer training of competitive snow skiers.
The idea of applying swivel casters to devices that simulate ski was disclosed as early as 1974 in U.S. Pat. No. 3,827,706 and revisited a decade ago in US Patent 2002195788 which discloses a wheeled device referred to as street-ski. The street-ski design has been commercially implemented as a board device known as T-board. The T-board turns in response to shifting the weight of the rider on the board towards the edge of the board. The casters mounted on the underside of the T-board are equipped with springs that resist pivoting and return the wheels to the straight position.
U.S. Pat. No. 7,195,259 discloses in FIG. 5a a board design with a leading swivel caster and two trailing fixed casters which is capable of performing carved turns. A similar design is disclosed in a product known in Japan as Ro-ski which comprises a short ski-like platform, leading swivel caster and two offset parallel fixed casters mounted at the rear of the platform.
Another device aimed at simulation of snow ski is disclosed in U.S. Pat. No. 7,784,833. It uses four fixed wheels mounted near the centre of a narrow platform and two swivel casters with springs that resist pivoting at both ends. However, since the casters are mounted in the same direction this device will not simulate carving.